This class provides the standard implementation for rendering an item (as in
a WAbstractItemView), and renders data provided by the standard data
roles (see ItemDataRole). It also provides default editing support using a
line edit.

The editor should not keep a reference to the model index (it does not
need to since
setModelData() will provide the proper model index to save the data to
the model). Otherwise, because model indexes may shift because of row or
column insertions, you should reimplement
updateModelIndex().

As an example of how to provide a specialized editor, consider the
default implementation, which returns a WLineEdit: